1. Field of the Invention
This present invention relates to a process for electric bonding of an aluminum wire to a contact area which wire provided with an oxide layer on its outer surface is wrapped with an insulating lacquer and ultrasonic bonded to said contact area.
2. Description of the Related Art
It has been a customary practice for many years to primarily use copper wires to make coils and windings for electric motors while the use of aluminum wires has been preferred in the recent past for reasons of cost, especially where production in large batches and under price pressure is involved such as for electric motors adopted in white goods (pump drives for dishwashers, laundry washing machines etc.). It has been found in this conjunction that other than copper wires such aluminum wires are not so well solderable and in particular not with adequate reproducibility and reliable performance quality. This means that the electric bonding of aluminum wires by way of soldering methods that have been customary for copper wires to date may create problems. It is however known to connect aluminum wires to a contact area (substrate) by ultrasonic bonding in that an aluminum wire is forced against said contact area under a defined pressure and ultrasonic welded. A factor of importance to the process here is the oxide layer surrounding the outer surface of the wire. This oxide layer constitutes a hard friction partner with the usually plane metallic substrate and hence improves the bonding ability of the aluminum wire.
The use of aluminum wire for coils and/or motor windings calls for the individual windings to be insulated against one another. This is the reason why the outer surface of the aluminum wire is wrapped with an insulating lacquer. Extensive tests have shown that said insulating lacquer has a strong negative effect on the bonding ability of an aluminum wire which when wrapped with insulating lacquer cannot be bonded to a contact area with adequate quality, in particular not in a reproducible, durable and reliable way.
It is an object of this present invention, therefore, to provide a process for removing insulating lacquer from selected areas of an aluminum wire which can be performed reliably and at low cost.
Applicant has initially attempted in a large number of test runs to remove insulating lacquer from aluminum wires by mechanical methods during which it was discovered that in all cases it was not only the insulating lacquer, but also the outer oxide layer of the wire that was removed or at least damaged. This oxide layer damage resulted in that the aluminum wire was less well to bond. Although the oxide layer used to regenerate as a rule after it had been removed or damaged, the time such regeneration takes is a drawback under process aspects.